fix(logger): print to stderr if log fails for default format (#2830)

We log to stderr if logging fails when a custom format is used, but not
for the default format. This change addresses this inconsistency.

middleware/logger/default_logger.go

@@ -2,6 +2,7 @@ package logger
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"sync"
 
@@ -60,7 +61,7 @@ func defaultLoggerInstance(c fiber.Ctx, data *Data, cfg Config) error {
 		}
 
 		// Write buffer to output
-		_, _ = cfg.Output.Write(buf.Bytes()) //nolint:errcheck // This will never fail
+		writeLog(cfg.Output, buf.Bytes())
 
 		if cfg.Done != nil {
 			cfg.Done(c, buf.Bytes())
@@ -92,15 +93,9 @@ func defaultLoggerInstance(c fiber.Ctx, data *Data, cfg Config) error {
 	if err != nil {
 		_, _ = buf.WriteString(err.Error()) //nolint:errcheck // This will never fail
 	}
+
 	mu.Lock()
-	// Write buffer to output
-	if _, err := cfg.Output.Write(buf.Bytes()); err != nil {
-		// Write error to output
-		if _, err := cfg.Output.Write([]byte(err.Error())); err != nil {
-			// There is something wrong with the given io.Writer
-			_, _ = fmt.Fprintf(os.Stderr, "Failed to write to log, %v\n", err)
-		}
-	}
+	writeLog(cfg.Output, buf.Bytes())
 	mu.Unlock()
 
 	if cfg.Done != nil {
@@ -129,3 +124,14 @@ func appendInt(output Buffer, v int) (int, error) {
 	output.Set(fasthttp.AppendUint(output.Bytes(), v))
 	return output.Len() - old, nil
 }
+
+// writeLog writes a msg to w, printing a warning to stderr if the log fails.
+func writeLog(w io.Writer, msg []byte) {
+	if _, err := w.Write(msg); err != nil {
+		// Write error to output
+		if _, err := w.Write([]byte(err.Error())); err != nil {
+			// There is something wrong with the given io.Writer
+			_, _ = fmt.Fprintf(os.Stderr, "Failed to write to log, %v\n", err)
+		}
+	}
+}

middleware/logger/logger_test.go

@@ -143,17 +143,17 @@ func Test_Logger_ErrorTimeZone(t *testing.T) {
 	require.Equal(t, fiber.StatusNotFound, resp.StatusCode)
 }
 
-type fakeOutput int
+type fakeErrorOutput int
 
-func (o *fakeOutput) Write([]byte) (int, error) {
+func (o *fakeErrorOutput) Write([]byte) (int, error) {
 	*o++
 	return 0, errors.New("fake output")
 }
 
 // go test -run Test_Logger_ErrorOutput_WithoutColor
 func Test_Logger_ErrorOutput_WithoutColor(t *testing.T) {
 	t.Parallel()
-	o := new(fakeOutput)
+	o := new(fakeErrorOutput)
 	app := fiber.New()
 	app.Use(New(Config{
 		Output:        o,
@@ -163,14 +163,13 @@ func Test_Logger_ErrorOutput_WithoutColor(t *testing.T) {
 	resp, err := app.Test(httptest.NewRequest(fiber.MethodGet, "/", nil))
 	require.NoError(t, err)
 	require.Equal(t, fiber.StatusNotFound, resp.StatusCode)
-
-	require.Equal(t, 1, int(*o))
+	require.EqualValues(t, 2, *o)
 }
 
 // go test -run Test_Logger_ErrorOutput
 func Test_Logger_ErrorOutput(t *testing.T) {
 	t.Parallel()
-	o := new(fakeOutput)
+	o := new(fakeErrorOutput)
 	app := fiber.New()
 	app.Use(New(Config{
 		Output: o,
@@ -179,7 +178,7 @@ func Test_Logger_ErrorOutput(t *testing.T) {
 	resp, err := app.Test(httptest.NewRequest(fiber.MethodGet, "/", nil))
 	require.NoError(t, err)
 	require.Equal(t, fiber.StatusNotFound, resp.StatusCode)
-	require.Equal(t, 1, int(*o))
+	require.EqualValues(t, 2, *o)
 }
 
 // go test -run Test_Logger_All
@@ -634,6 +633,13 @@ func Test_Logger_ByteSent_Streaming(t *testing.T) {
 	require.Equal(t, "0 0 200", buf.String())
 }
 
+type fakeOutput int
+
+func (o *fakeOutput) Write(b []byte) (int, error) {
+	*o++
+	return len(b), nil
+}
+
 // go test -run Test_Logger_EnableColors
 func Test_Logger_EnableColors(t *testing.T) {
 	t.Parallel()
@@ -646,6 +652,5 @@ func Test_Logger_EnableColors(t *testing.T) {
 	resp, err := app.Test(httptest.NewRequest(fiber.MethodGet, "/", nil))
 	require.NoError(t, err)
 	require.Equal(t, fiber.StatusNotFound, resp.StatusCode)
-
-	require.Equal(t, 1, int(*o))
+	require.EqualValues(t, 1, *o)
 }